The continuous measurement of blood pressure by use of arterial tonometer transducers is well known as shown in U.S. Pat. Nos. 3,123,068 to R. P. Bigliano, 3,219,053 to G. L. Pressman, P. M. Newgard and John J. Eige, 3,880,145 to E. F. Blick, 4,269,193 to the present inventor, and 4,423,738 to P. M. Newgard, and in an article by G. L. Pressman and P. M. Newgard entitled "A Transducer for the Continuous External Measurement of Arterial Blood Pressure" (IEEE Trans. Bio-Med. Elec., April 1963, pp. 73-81).
In a typical tonometric technique for monitoring blood pressure, a transducer which includes an array of pressure sensitive elements is positioned over a superficial artery, and a hold-down force is applied to the transducer so as to flatten the wall of the underlying artery without occluding the artery. The pressure sensitive elements in the array typically have at least one dimension smaller than the lumen of the underlying artery in which blood pressure is measured, and the transducer is positioned such that at least one of the individual pressure-sensitive elements is over at least a portion of the underlying artery. The output from one of the pressure sensitive elements is selected for monitoring blood pressure. In some prior art arrangements, the pressure sensitive element having the maximum pulse amplitude output is selected, and in other arrangements the element having a local minimum of diastolic or systolic pressure which element is within substantially one artery diameter of the element which generates the waveform of maximum pulse amplitude is selected.
The pressure measured by the selected pressure sensitive element, i.e. the element centered over the artery, will depend upon the hold-down pressure used to press the transducer against the skin of the subject. Although fairly accurate blood pressure measurements are obtained when a hold-down pressure within a rather wide pressure range is employed, it has been found that there exists a substantially unique value of hold-down pressure within said range for which tonometric measurements are most accurate. This so-called "correct" hold down pressure varies among subjects. With prior art tonometric type transducers the correct hold-down pressure often is not determined thereby leading to inaccuracies in the blood pressure measurements.